Islamabad, Pakistan - A roadside bomb targeting a bus has killed at least 14 people in the northwestern Pakistani district of Kurram, local officials say.

The explosion occurred in central Kurram as the vehicle travelled to the town of Sadda, about 250km west of the capital Islamabad, early on Tuesday morning, Majidullah, a local administration official, told Al Jazeera.

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"There were about 23 passengers in the van, and there was a bomb placed on the roadside near the village of Godar, he said.

"Security forces have now initiated a search operation in the area."

Officials said that at least five women and four children were among the 14 killed, and at least 10 people were wounded in the explosion.

Kurram lies along Pakistan's northwestern border with Afghanistan, and has seen a series of attacks in recent days, often targeting the district's sizeable Shia Muslim minority.

The Pakistani Taliban's Jamaat ur Ahrar faction has claimed responsibility for the attack.

The Pakistani Taliban claimed responsibility for both of those attacks. Pakistan has been battling the group - an umbrella organisation of armed religious groups - since 2007, and has launched a series of military operations against them, including one in Kurram district that concluded in 2011.

In response to the latest spate of attacks, Kurram's residents have formed tribal militias to work with Pakistan's security forces to secure the area. They have dug trenches around Parachinar, as well as establishing bunkers at strategic locations.

This area, however, had no increased security, according to local official Majidullah.

"This area is in central Kurram - there is no increased security there," he said.

Pakistan began the second round of a countrywide population census on Tuesday, with more than 84,000 census workers dispatched to 87 districts across the country. It is Pakistan's first census since 1998, and the military has deployed more than 200,000 troops to provide security for the exercise.